how should we do
?no id is returned more than once (name of the test method: idsAreUnique)
Are testing 1.000.000 calls enough?
This function will fail as the numbers can not be increased infinitely.
Should the test work only in this case.
Generally solutions with storing all results will fail because of storage/heap/memory overflow depending on the number of calls.
Specific solutions may take a long time.
Here would be a proof good which is saying it would not fail for infinite data types.